The present invention generally relates to a temperature sensor assembly and, in particular, relates to such an assembly adapted for use in conjunction with a molecular beam epitaxial growth effusion cell.
In the general field of semiconductor manufacturing one of the most technologically sophisticated systems is known as the molecular beam epitaxial growth system (hereinafter referred to as MBE or an MBE system). In very simplified terms an MBE system is one in which thermally excited atoms, or molecules, of one or more materials, for example a dopant, are produced in effusion cells and, as beams, bombard a semiconductor substrate. By bombarding the substrate in an accurate and selective fashion, well defined layers of various compositions are formed on the substrate. These well defined layers then serve as the essential structure for fabrication of semiconductor devices. The thickness of these layers can, by, inter alia, computer controlled mechanisms, be very accurately controlled thus resulting in well defined device structures. As one skilled in the semiconductor art will recognize, a critical factor in the fabrication of a semiconductor device on a substrate, is in the depth composition, or dopant profile, of the layered structure. Ideally, in most instances, the composition should be uniform throughout a particular layer. MBE systems appear capable of producing structures consisting of well defined and abrupt interfaces. It is in furtherance of this goal that the present temperature sensor assembly is directed.
One of the most critical components of an MBE system is the effusion cell. In general, an effusion cell is the source of the atomic, or molecular, beam. Usually a material is placed in the effusion cell, which is effectively a crucible of refractory material, and heated to a temperature at which a beam of atoms, or molecules, are emitted. The beam fluxes, i.e., the cross-sectional density of atoms, or molecules, impinging upon the substrate directly determines the composition and growth rate for each molecular, or atomic, layer of the structure.
The beam flux impinging on the substrate is directly proportional to the vapor pressure of the dopant in the effusion cell. The vapor pressure is dependent upon the temperature of the cell. In fact, a 0.5.degree. C. change in the cell temperature typically results in a 1% change in the beam flux. Thus, it is profoundly clear that if various devices' or more specifically, the composition and thickness of the various layers, are to be reproducible it is imperative to be able to very accurately maintain a constant cell temperature and to be able to reproduce that same temperature.
Conventionally, the effusion cell temperature is monitored by means of a thermocouple junction placed either under or on the side of the crucible. Both positions are quite poor, and consequently, it is not presently possible to accurately monitor and reproduce a given effusion cell temperature. Specifically, if the thermocouple is positioned under the crucible the dominant mechanism for heat reaching the junction is by conduction transfer from the bottom of the crucible. However, thermal conduction between any two adjacent materials is directly related to the actual force working to maintain contact. In such a case then, the thermal conduction between two adjacent materials is directly related to the pressure therebetween. In the present case, the pressure is, for all intents and purposes, the combined weight of the crucible and its contents. However, as the contents escape by vapor the pressure between the crucible and the junction is reduced. Consequently, the thermal conduction varies and the temperature reading varies. Hence, it is a rather inaccurate method. As a further inherent error of the above-described positioning problem, significant cooling of the thermocouple junction itself occurs when the conducted heat into the junction is small because heat lost through the wires of the junction is relatively large.